Rotatable waveguide joint



Jan. 3, 1961 J. T. FRASER ROTATABLE WAVEGUIDE JOINT Filed June 13, 1958 INVENTOR.

JULIUS T. FRASER ATTORNEY.

United States Patent O ROTATABLE WAVEGUIDE JOINT Julius T. Fraser, Pleasantville, N.Y., assignor to General This invention relates to rotatable joints in microwave waveguides.

In applying microwave energy to rotating or oscillating apparatus, such as circular and sector scan radars, it becomes necessary to transfer the energy from a fixed waveguide, usually rectangular in cross section, to a rotating rectangular waveguide. The present invention accomplishes this in a convenient and efficient manner. The microwave energy in the fixed rectangular waveguide is transferred to a round waveguide where a polarized mode of microwave propagation is excited, the TE being one of several such modes that can be used. The round waveguide contains a mechanical rotatable joint and in addition contains a polarization rotator which twists the polarization of the microwave energy through the same angle as the mechanical rotation. The rotated microwave field is transferred to the moving rectangular waveguide. An arrangement is provided to drive the microwave polarity rotating device from the rotating joint.

One convenient polarity-rotating device now in com mon use employs a substance, such as a magnetized ferrite, to produce what is sometimes termed Faraday magneto-optical rotation of a polarized microwave field. Such a device is employed in the apparatus of the invention. The adjustment of the device is effected by controlling the degree of magnetization induced in it by a permanent magnet through control of the reluctance of the magnetic circuit.

When the device of this invention employs a ferrite rotator the device is nonreciprocal, and for a given sense of drive coupling between the mechanical members of the rotatable joint and the adjustable ferrite rotator the direction of power transmission must be in a specific direction.

The device of this invention provides a compact, broadband rotating joint which has no sliding contacts. Insertion loss and standing waves are low with proper terminal matching.

The principal purpose of this invention is to provide a microwave rotatable waveguide joint containing a mechanical joint and a polarity-rotating device acting in concert.

Another purpose of this invention is to provide a round waveguide containing a device for changing the polarization direction of microwave energy passing through it, in combination with a mechanical rotatable connection.

Still another purpose is to provide a waveguide including a rotatable mechanical connection between two round waveguides, a polarity-rotating device in one of the round waveguides and a permanent magnet source of magnetization coupled through adjustable magnetic paths to the polarity-rotating device.

A further understanding of the invention may be secured from the detailed description and associated drawings, in which:

Fig. 1 depicts an embodiment of the invention employing rotatable pole pieces.

Fig. 2 illustrates a pole piece suitable for use in the embodiment of Fig. l. I

through its waveguide.

Fig. 3 depicts a second embodiment in which the dis tance of the permanent magnet from the ferrite rotator is varied.

Referring now to Fig. 1, a rectangular input waveguide 11 is connected to the end of a round waveguide 12. The transition between the two waveguides is preferably of a nature to prevent microwave energy reflections. For example, as more fully described in Patent 2,758,282, if the round waveguide be filled or partly filled with solid dielectric material then the two waveguides, if of proper sizes, can be connected directly without producing any impedance discontinuity. Otherwise, matching irises or posts may be employed or a taper section inserted.

The round waveguide 12 is joined to a similar round waveguide 13 by a rotatable mechanical joint 14 of conventional design which may include a pressurization gasket and an annular choke recess to prevent escape of microwave energy and reduce transmission loss. The end 16 of round waveguide 13 is connected to an output rectangular waveguide 17, with suitable impedance matching.

In some circuit designs it may be desired to connect either or both of the rectangular waveguides to the roundv waveguide section by right-angled transitions. Such a conventional transition may be employed in place of either of the described end-to-end transitions, the rectangular waveguide making a matched H-plane or E-plane sidearm connection to the round waveguide.

The round waveguide 13 contains a long, thin rod 18 which is capable of producing Faraday magneto-optical rotation of a microwave field passing through the waveguide. The rod is preferably of ferrite of the type generally used in microwave rotators, phase changers and isolators. One ferrite which may be employed, for example, is known as Ferramic R-1, and is made by the General Ceramics Corporation of Keasby, NJ. This material has a composition of 70 moles of MgO, 8 moles of MnO and 22 moles of Fe O The ferrite rod 18 is axially positioned in the round waveguide 13 by means, for example, of two thin dielectric discs 19 and 21.

The ferrite rod 18 may be surrounded, if desired, by a dielectric sleeve 22, or the interior volume of the round waveguide may be completely filled in the region of the ferrite rod by solid dielectric, in which case the discs 19 and 21 are not needed.

A ferrite rod such as described, when magnetized, rotates the plane of polarization of microwave energy passed Magnetization of the rod is accomplished in this device by positioning a permanent magnet 23 so that a part of its magnetic field passes longitudinally through the ferrite rod 18. The permanent magnet is secured in a fixed position relative to the fixed input waveguides 11 and 12 and parallel to the common axis of the waveguides 12 and 13 by means not shown but indicated by the dashed line 24. Since this line 24 indicates that the permanent magnet 23 is fixed in position relative to the short round waveguide 12, the waveguide 13, when rotated, moves relative to the permanent magnet 23.

In order to bridge and control the air gaps between the ends of the permanent magnet 23 and the ends of the ferrite rod 18 in approximate juxtaposition thereto, two soft iron pole pieces 26 and 27 are provided. Each pole piece is preferably grooved into the outer surface of waveguide 13 so that the inner circumference of the pole piece is but little larger than the inside dimension of the waveguide, to minimize the airgaps between the pole pieces and the ferrite rod. Alternatively the pole piece inner surfaces may be flush with and continuous with the inner surface of the waveguide, in which case the iron pole pieces must be conductively joined to the material of the waveguide.

as identical in shape. The inner circumference 28 is circular to fit the waveguide. The outer periphery 29 varies in its distance from the inner circumference 28 over 360 in accordance with some function, such as an Archimedean spiral. Other functions may be employed inorderto secure the desired mode of operation. It may be convenient in some cases to make the two pole pieces of two different shapes so that their combined effect produces the desired magnetic variation. In order to make the radial part of the 360 pole piece produce a more abrupt change in reluctance the lip 29' is preferably undercut. One end of the permanent magnet 23 is illustrated to depict its position closely adjacent to the pole piece but spaced from the peripheral surface .29 at all rotational positions.

In the operation of the embodiment of Fig. 1, the dominant TE mode of microwave energy in the rectangular waveguide 11 excites the dominant TE mode "in the round waveguide 12, the energy in this mode passing on into the waveguide 13. Let it be assumed that the round waveguide 13 has been rotated counterclockwise relative to round waveguide 12 so that the output rectangular waveguide 17 has been rotated about 100 from an initial position collinear with the input waveguide 11. In this rotated position, if the ferrite rotator were not present unrotated microwave field would enter waveguide 17 but little; most would be reflected back toward the source. However, in this device the poling of the permanent magnet 23 is in such sense and the mag netic field induced thereby is of such strength as to produce counterclockwise rotation of the microwave energy polarization passing through the round waveguide 13. This rotation is in the same sense as the mechanical rotation of waveguides 13 and 17 relative to waveguides 11 and 12, and in the same amount, here suggested as about 100. Therefore the field polarization applied to theinput end of waveguide 17 is such as to be accepted thereby. No energy is reflected but all is converted into I the TE rectangular waveguide mode.

The magnetic change induced in the ferrite rod as the pole pieces are rotated is obviously not a linear function of the lengths of air gaps for several reasons, one being that rotation not only changes the amount of airgap in the closed magnetic circuit consisting of the permanent magnet, the two pole pieces and the ferrite rod, but also varies the amount of leakage flux which flows from one pole of the permanent magnet 23 directly to the other polethereof. It is obvious, therefore, that a pole piece shape departing somewhat from an Archimedean spiral may be necessary to make the relation between field rotation and mechanical joint rotation a linear one. It is also obvious that the effect of rotating the pole pieces is greater than that caused by change of magnetic circuit reluctance alone, due to the leakage flux variation. Since the polarization rotation changes continuously in the same direction for 360, this embodiment can be used for continuous circular scanning.

A second embodiment also includes a permanent magnet as depicted in Fig. l, but the pole pieces 26 and 27 are eliminated and the magnetic induction in the ferrite rod is varied by bodily moving the permanent magnet 23 radially, thus changing its distance from the ferrite rod 18. This is done by substituting a geared eccentric iarrangement for the fixed support indicated by the dashed ine 24.

One such arrangement is indicated in Fig. 3. A cylinder 31, which may be skeletonized, somewhat larger than the round waveguides 12 and 13, surrounds these waveguides so that the cylindrical axis is parallel to the waveguide axis but is not coincident with it. This relation is shownin the cross section of Fig. 3, in which the round waveguide 13 containing its ferrite rod 18 is surrounded by the larger cylinder 31 having its center atthe point 36. This cylinder is rotatably supported from the structure of the round waveguide 12, Fig. 1, and; also at the other end from the wavegt id 3 at or near its end 16. At this point the waveguide 13 is provided with an external ring gear meshing with an internal ring gear mounted on the inside of cylinder 31, Fig. 3. The pitch circumferences of these two ring gears are indicated by the dashed lines 32 and 33, respectively. A permanent magnet of U-shape similar to the shape of permanent magnet 23, Fig. 1, is mounted inside the large cylinder, one pole 34, Fig. 3, thereof being depicted.

In operation, when the permanent magnet pole 34 is in the position shown it is at its maximum distance from the ferrite rod 18. When, however, the round waveguide 13 is rotated in either direction, for example counterclockwise, the gear 32 thereon drives the large cylinder gear 33 counterclockwise. This notonly transports the permanent magnet counterclockwise about the large cylinders center of rotation 36 but also carries it closer to the round waveguide 13 until the magnet touches the round waveguide as indicated at the dashed position 34'. The drawing shows a rotation of about but by change of the mechanical proportions can be secured. However, this is the maximum angle of joint rotation for this embodiment. It is useful in connection with sector scanning but cannot be used in circular scanning.

What is claimed is:

1. A microwave rotatable waveguide joint comprising, a first circular waveguide section, a second circular waveguide section, a rotatable joint connecting said first and second circular waveguide sections in axial alignment, means for passing microwave energy in the dominant TE mode through said circular waveguide sections, a ferrite rod capable when magnetized of rotating microwave energy, said ferrite rod being positioned axially within said first circular waveguide section, a permanent magnet emitting a magnetic field positioned to produce a magnetic field in said ferrite rod longitudinally thereof, said permanent magnet being positioned out side said first circular waveguide section, the poles of said permanent magnet being respectively juxtaposed to the ends of said ferrite rod, a pair of highly permeable pole pieces secured to the outside of said first circular waveguide section respectively between the poles of said permanent magnet and the ends of said ferrite rod, said pole pieces having internal surfaces concentric with said circular waveguide section and being provided with an external periphery eccentric with respect thereto, and means securing said permanent magnet to said second circular waveguide section whereby relative motion between said first and second circular waveguide sections varies the reluctance of the magnetic circuit between said permanent magnet and said ferrite rod in such sense and amount and at such rate that at any rotational angle between said first and second circular waveguides the microwave energy polarization is rotated by the same angle.

2. A microwave rotatable Waveguide joint in accordance with claim 1 in which the external periphery of each of said pole pieces has a fiat spiral form extending over substantially 360 combined with a substantially radial portion joining the ends of the spiral form.

3. A microwave rotatable waveguide joint comprising, a first circular waveguide section, a second circular waveguide section, a rotatable joint connecting said first and second circular waveguide sections in axial alignment, means for passing microwave energy in the dominant TE mode through said circular waveguide sections, a ferrite rod capable when magnetized of rotating microwave energy, said ferrite rod being positioned coaxially within said first circular waveguide section, a permanent magnet emitting a magnetic field positioned to induce a magnetic field in said ferrite rod longitudinally thereof whereby the. direction of the polarization of-said microwave energy is rotated, said permanent magnet being positioned outside said first circularwaveguide section,

the poles of said permanent magnet :being juxtaposedito,

the respective ends of said ferrite rod, a gear surrounding said second circular waveguide section and secured thereto, an internal gear larger than said gear and meshing therewith, and means securing said internal gear to said permanent magnet whereby relative motion of said first and second circular waveguides moves said permanent magnet radially relative to said ferrite rod whereby the strength of the magnetic field applied to said ferrite rod by said permanent magnet is varied in such sense and amount and rate as to make the microwave field polarization rotation caused by said magnetized ferrite rod at any relative angle of said first and second circular waveguides to be equal to that relative angle.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Mar. 14, 1956 

